It is known to manufacture hollow metallic articles by diffusion bonding and superplastic forming metal workpieces. These metal workpieces include elementary metal, metal alloys, intermetallic materials and metal matrix composites.
The diffusion bonding and superplastic forming process may be used to produce contoured articles for example fan blades, or fan duct outlet guide vanes, for gas turbine engines by superplastically, or hot forming, an integral structure formed by the diffusion bonding process.
A procedure for manufacturing an article by diffusion bonding and superplastic forming is disclosed in our European patent EP0568201B. In EP0568201B the integral structure formed by the diffusion bonding process is twisted before the integral structure is superplastically formed. Additionally the integral structure is hot creep formed in the superplastic forming dies.
There is a requirement to increase the thrust produced by gas turbine engines. This has lead to an increase in the size of the fan duct, or bypass duct, of the gas turbine engine with a consequential increase in the size of the fan blades and fan duct outlet guide vanes, in particular an increase in the length of the fan blade from root to tip.
The above mentioned manufacturing process consistently results in satisfactory fan blades and/or fan duct outlet guide vanes of present finished size, for example a length of approximately 0.9 m from root to tip for a fan blade. However, it has now been found that the above mentioned manufacturing process does not consistently result in satisfactory fan blades and/or fan duct outlet guide vanes of increased finished size, for example a length of approximately 1.0 m from root to tip for a fan blade.
It has been found that the diffusion bond produced between the metallic workpieces is unsatisfactory for the fan blades of increased size. The diffusion bond between the metallic workpieces forming a fan blade must be of very high quality in order to withstand the high stresses that the fan blade experiences in operation. It has been found that satisfactory diffusion bonds were not being consistently produced because all the air and any impurities between the metallic workpieces was not removed before the diffusion bonding process occurred. It is essential to diffusion bond the metallic workpieces with a vacuum therebetween in order to obtain a high quality diffusion bond.